Light emitting diode scanner

ABSTRACT

A portable scanning head emits and receives light from a light emitting diode to read symbols, such as bar-code symbols. The optics within the scanner are operative for focusing a light beam and the view of a light sensor in different planes exteriorly of a scanner housing. Imaging means are provided in the unit for imaging a viewing window. The viewing window has an area smaller than that of the scan spot. The system can employ an LED as a light source and tolerate the relatively large-sized (on the order of millimeters) scan spot without sacrificing reading performance since the photodiode &#34;sees&#34; only that portion of the scan spot visible through the viewing window.

RELATED CASE

This application discloses subject matter also disclosed in ourcopending application Ser. No. 562,130, filed Aug. 3, 1990.

BACKGROUND OF THE INVENTION

This invention generally relates to an electro-optical scanning systemfor reading symbols, especially bar code symbols and, more particularly,to non-laser-based scanners operative for focusing a light beam and theview of a light sensor in different planes exteriorly of a scannerhousing.

It has heretofore been proposed to read bar code symbols, particularlyof the Universal Product Code (UPC) type, by using laser and non-laserscanners of the type disclosed in, for example, U.S. Pat. Nos.4,251,798; 4,387,297; 4,409,470; 4,806,742 and 4,825,057, all of whichhave been assigned to Symbol Technologies, Inc., the assignee of thisinvention, and are hereby incorporated by reference herein.

Typically, a laser beam generated by a laser source, for example, a gaslaser tube or a semiconductor laser diode, is optically focused by anoptical train into a generally circular laser beam spot on a symbol. Thebeam spot is swept by a scanning component over the symbol and forms ascan pattern thereon. Laser light reflected off the symbol is detectedby a light sensor, e.g. a photodiode, mounted together with the lasersource, the optical train, the scanning component, and the photodiode ina housing, preferably one having a handle to enable hand-held, portableoperation.

The symbol itself is a coded pattern comprised of a series of bars ofvarious widths, the bars being spaced apart from one another to boundspaces of various widths, the bars and spaces having differentlight-reflective properties. Although dimensions may vary, depending onthe particular application and the density of the symbol, each bar andspace of a UPC symbol typically used in the retail industry to identifyretail products measures on the order of thousandths of an inch (mils).In practice, the generally circular laser beam spot has across-sectional diameter on the order of 6 to 10 mils.

Although the known laser scanners have enjoyed considerable commercialsuccess, there is nevertheless incentive to reduce the cost of thescanner unit. The laser devices produce a very intense light spot ofsmall size, and thus have inherent advantages. However, the laser lightsources are of relatively high cost compared, for example, to non-lasersources such as light emitting diodes (LEDs). The use of non-lasersources presents problems, since it is difficult to focus anon-collimated LED source to beam spot sizes measuring on the order ofmils, at least not without resorting to expensive, heavy,multiple-element optical trains or loss of power. LEDs can typically befocused to spot sizes on the order of millimeters. However, using such alarge-sized beam spot to read bars and spaces which measure on the orderof mils imposes a significant burden on the signal processing and decodecircuitry for the scanner. Non-reads and reading errors are likely.

By contrast, in laser-based systems, where the laser beam spotdimensions are on the same order of magnitude as those of the bars andspaces to be decoded, the signal processing and decoding circuitry hasno such burden. The photodiode in such laser-based systems typically"looks" at a large volume of space surrounding the beam spot and in acommon plane therewith.

SUMMARY OF THE INVENTION

It is a principal object of this invention to provide improvedelectro-optical scanners. Another object is to provide improvements inpracticality, reliability, freedom from errors, and/or cost reduction inlow-cost, non-laser-based scanners. An additional object is to provide amethod of focusing a light beam into a scan spot, and to focus the viewof a photodiode into a viewing window which is sized to lie within thescan spot. A further object is to provide a method of positioning thescan spot and the viewing window in different planes exteriorly of ahousing for the scanner. In addition, an object is to provide a scannerdevice which is of low cost and high reliability, and is simple andconvenient to operate by unskilled users. Further, an object is toprovide an improved scanner that automatically adapts to varyingscanning conditions without operator intervention.

According to one embodiment of the invention, in keeping with theseobjects, and others which will become apparent hereinafter, one featureresides, briefly stated, in a system for, and a method of, readingsymbols using an LED type of light source. The system includes a lightsource means, e.g. an LED, for generating a non-laser light beam.Focusing means are provided in the unit for focusing the light beam to ascan spot of predetermined area at a scan plane that is locatedexteriorly of the unit. The scan spot is positioned on a symbol locatedin a working distance range in the vicinity of the scan plane.

The system further includes scanning means, e.g. a motor-driven mirror,for sweeping the scan spot across the symbol in a scan. Thus, light isreflected off the symbol in all directions. At least a returning portionof the reflected light travels away from the symbol back toward theunit. This returning portion has a variable intensity over the scan.

Sensor means, e.g. a photodiode, in the unit is operative for viewingand detecting the variable intensity of the returning portion of thereflected light over a field of view. The photodiode generates anelectrical signal indicative of the detected variable light intensity.Signal processing means processes the electrical signal and generatestherefrom data descriptive of the symbol.

In accordance with one feature of this invention, imaging means areprovided in the unit for imaging a viewing window. The viewing windowhas an area smaller than that of the scan spot. The viewing window ispositioned at a viewing plane that is spaced away from the scan plane.Thus, the system can employ an LED as a light source and tolerate therelatively large-sized (on the order of millimeters) scan spot withoutsacrificing reading performance since the photodiode "sees" only thatportion of the scan spot visible through the viewing window. The viewingwindow is configured to have dimensions on the order of mils, so thephotodiode "sees" only a very small central portion of the scan spot.Hence, the photodiode no longer "sees" a broad volume of spacesurrounding the scan spot, but, instead, "looks" at a very small volumeof space within the scan spot.

In a preferred embodiment, the focusing means includes a focusing mirroroperative to optically form the scan spot to have a generally circulararea. Preferably, the focusing mirror has a generally sphericalreflecting surface.

Also, the imaging means includes an apertured wall having an apertureadjacent a sensor opening of the photodiode or a very small photodiode.Reflected light passes through the aperture en route to the photodiode.That is, the aperture defines the sensitive area of the sensor.

The imaging means further includes an imaging mirror, preferably havinga generally spherical reflecting surface. Advantageously, both thefocusing mirror and the imaging mirror are mounted in the unit for jointmovement by the scanning means longitudinally across the symbol. Thescanning means includes a reciprocatingly oscillatable scanningcomponent on which both mirrors are mounted for joint movement about anaxis.

The imaging mirror is mounted on the focusing mirror, and is alsoangularly offset relative to the axis. In a preferred case, wherein thelight beam is directed along a first optical axis, and wherein theviewing window of the sensor means is imaged by the imaging means alonga second optical axis, the angle included between said optical axes istwice the angular offset between the imaging and focusing mirrors.

The housing may have a handle to enable hand-held operation. Thehand-held housing or head is aimable by a user at the symbol to be readwhich is in contact with or at a longitudinal distance from a front wallof the head. This device is optimized for contact use: here the opticalpath is almost entirely within the housing. In one embodiment, the scanplane and the viewing plane are offset relative to one another, near orat the front wall of the housing.

In another embodiment, the sensor means may include a pair ofphotodiodes for increased zone coverage, each photodiode having a sensoropening. In this case, the imaging means includes a pair of aperturedwalls, each having an aperture situated adjacent to, or constituting, arespective sensor opening. The imaging means is operative, in this case,for imaging each viewing window of the respective photodiodes to a pairof viewing windows positioned in different viewing planes exteriorly ofthe housing. In a preferred application, the "closer-in" viewing planemay be used in the decoding of high-density symbols, whereas, the"further-out" viewing plane may be used in the decoding of low-densitysymbols.

According to a feature of another embodiment, the characteristics of thebar code symbols being read are detected, with respect to beinghigh-density or low-density bar code symbols, and the signal processingcircuitry is altered in bandwidth in response to this detection. In thismanner, the bandwidth best suited for reading each type of bar code isutilized by the processing circuitry. A wide bandwidth will allow morenoise to propagate through the processing circuitry, and so thebandwidth is narrowed to use only that which is needed. Low density barcode do not require as much bandwidth in the signal processing circuitryas high-density bar codes.

In order to lower the cost of the bar code reader device, mechanical andelectrical parts are eliminated whenever possible. In one embodiment,the trigger switch ordinarily used in a hand-held scanner is eliminatedby using another technique for turning the unit on when the user isready to read a bar code. A motion sensor detects when the user picks upthe hand-held unit, and turns on the light source and scanner motor inresponse to this detection. The motor and light source can then beturned off by a time-out circuit, or in response to decoding a valid barcode.

BRIEF DESCRIPTION OF THE DRAWINGS

The novel features which are considered as characteristic of theinvention are set forth in the appended claims. The invention itself,however, both as to its construction and its method of operation,together with additional objects and advantages thereof, will be bestunderstood from the following description of specific embodiments whenread in connection with the accompanying drawings, wherein:

FIG. 1 is an elevation view in diagrammatic form of one embodiment of anon-laser-based system for reading bar code symbols according to thisinvention;

FIG. 2 is an enlarged view of a viewing window superimposed on a scanspot that, in turn, is superimposed over a bar code symbol to be read bythe system of FIG. 1;

FIG. 3 is an elevation view of a scan motor and mirror assembly used inthe scanner of FIG. 1;

FIG. 4 is a top view of the scan motor and mirror assembly of FIG. 3;

FIG. 5 is an elevation view of a hand-held scanner unit constructedaccording to the embodiment of FIGS. 1, 3 and 4;

FIG. 6 is an electrical circuit diagram in schematic form of a digitizercircuit used in the system of FIGS. 1-5 according to one embodiment ofthe invention;

FIGS. 7a to 7f are timing diagrams of voltages appearing in the circuitof FIG. 6;

FIG. 8 is an electrical circuit diagram in schematic form of a movementdetector circuit used in the system of FIGS. 1-5 according to a featureof an alternative embodiment of the invention;

FIG. 9 is an elevation view in diagrammatic form (corresponding toFIG. 1) of another embodiment of a non-laser-based system for readingbar code symbols;

FIG. 10 is an enlarged view of a bar code symbol with two viewingwindows and a scan spot (corresponding to FIG. 2) for the embodiment ofFIG. 9;

FIG. 11 is a top view of a currently preferred commercial embodiment ofthe scanner head in accordance with this invention;

FIG. 12 is a side view of a currently preferred commercial embodiment ofthe scanner head in accordance with this invention; and

FIG. 13 is an electrical circuit diagram in schematic form of adigitizer circuit that may be used in the system of FIGS. 1-5 accordingto an embodiment of the invention.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

Referring now to FIG. 1 of the drawings, a system for reading symbols isillustrated. The term "symbol" as used herein is intended to coverindicia composed of portions having different light-reflectiveproperties. The indicia may, in a preferred case, be the omnipresent UPCbar code symbol, or other codes such as Code 39, Codeabar, Interleaved 2of 5, or other characters.

The system of FIG. 1 includes a non-laser type of light source 10, suchas an LED or semiconductor light emitting diode. The source 10 producesa non-coherent, non-collimated, wide-angle light beam 11 which isfocused by a curved mirror 12 onto a plane 13. The bar code symbol 14 tobe read is shown in FIG. 2, and it is noted that a spot 15 of lightproduced by the focused beam 11 is much larger than the minimumdimension 16 of the bars or spaces of the symbol 14 in the viewingplane. A light sensor 17 such as a photodiode is included in the systemof FIG. 1, and the light reflected from the symbol 14 due to light beam11 is focused by a smaller curved mirror 18 onto the detecting surfaceof this sensor. An apertured wall 19 blocks reflected light fromreaching the sensor 17 except that passing through a slit 20. This slitis preferably of an elliptical shape, perhaps 6×16 mils in size, todefine a field view or viewing window 21 for the sensor as seen in FIG.2. The viewing window 21 of the sensor 17 is focused by the mirror 18 tobe in a plane 22, which is axially spaced from the focal plane 13produced by the mirror 12 for the light beam 11. The symbol 14 to beread is located in the plane 22, so the image of the slit 20 in theplane 22 forms the viewing window 21 on the symbol. The focal lengths ofthe two mirrors 12 and 18 are the same in an example embodiment, so thespacing between plane 13 and plane 22 is due to the difference inspacing of the light source 10 and the sensor 17 from the mirrors.

The mirrors 12 and 18 are driven by a motor 23 as seen in FIGS. 3 and 4so that the spot 15 and the viewing window 21 move in the plane 22across the symbol 14 in a scan line 24 as seen in FIG. 2. The smallermirror 18 is mounted on the larger mirror 12 and is angularly displacedin a manner such that a central axis of the mirror 18 is displaced by anangle α with respect to a central axis of the mirror 12. The lightsource 10 is on an axis with respect to the mirror 12 which is angularlydisplaced, by an angle β, with respect to an axis on which the sensor 17resides. The angle α is one-half that of angle β.

The scanned spot 15 resulting from the light beam 11 is much larger thanthe viewing window 21 of the sensor 17, so only a very small part of theillumination by the light beam is sensed; of course, only a small partof the reflected light reaches the photodetector. In addition, an LEDgenerally produces a light beam of lower intensity compared to a lasersource. Thus, it is important that the mirror 12 be large so that moreof the LED light is focused onto the scan plane and the light density inthe spot 15 is reasonably high. Note that the construction of a typicallaser scanner is reversed from that of FIGS. 1 and 2; in a laser scannera bright, sharply-focused laser beam produces a spot of about the sizeof the minimum dimension 16 of the bar code pattern, then thephotodetector employed has a field of view much larger than the laserbeam spot. In contrast, here the spot 15 produced by the light beam islarge, and the viewing window 21 is small.

Referring to FIG. 5, in one embodiment, the scanning system of FIGS. 1-3is mounted in a hand-held, gun-shaped housing 25 which has a handle 26and a barrel portion 27. The system is, in this embodiment, a "contact"type of bar code scanner, in that the package containing the bar codesymbol 14 is directly in front of the front or snout end of the housingwhen the reading operation takes place. A window or opening 28 in thefront end of the barrel portion 27 of the housing 25 is transparent tothe light beam 11 and the reflected LED light going back to the sensor17, and this window 28 is placed in contact, or very close to, thesurface 29 of the package having the bar code symbol 14 thereon. Anactual window element may be omitted (i.e., it may be just a hole formedin the housing), or the window 28 may be spaced a distance such as 1/2inch inside the front end of the housing; this spacing protects thewindow from scratching.

The plane 22 of the image of the slit 20 is seen to be almost in thesame plane as that of the light sensor 17, as the sensor is near thefront of the barrel 27, vertically displaced from the window 28. Themirrors 12 and 18 and the drive motor 23 are mounted in the back end ofthe barrel 27. The circuitry used to process the electrical signalproduced by the sensor, and other control circuitry needed in the unit,is mounted on circuit boards 30 in the handle 26. The unit of FIG. 5 isconnected to a terminal or base station by a cable 31 containingconductors for carrying the processed bar code data, as well as voltagesupplies for the light source 10 and motor 23. Alternatively, an RF linkmay be used to send the signals back to a base station, in which case abattery is included within the housing 25, e.g., in the handle. Ineither event, a compact, light-weight, hand-held unit is thus provided.

FIG. 6 depicts a preferred digitizer; i.e., a circuit to obtain adigital signal representing a bar code from the analog signal developedby the light sensor or photodetector 17. The sensor 17 is connected tocircuitry for processing the electrical signals produced as a result ofthe bar code scan. FIGS. 7a to 7f depict the signals appearing atvarious points in the circuit of FIG. 6.

FIG. 7a depicts the original analog signal that the sensor 17 feeds to acurrent-to-voltage converter 32. The converter 32 feeds the developedvoltage to a differentiator 33. The differentiator 33 develops thefirst-derivative of the converted analog signal. The differentiator 33sends this first-derivative signal to an amplifier 34 which amplifiesand filters the first-derivative signal. The solid line FIG. 7b depictsthis amplified and filtered (and inverted) first-derivative signal. Thissignal is inverted since each of the converter 32, the differentiator33, and the amplifier 34 receives its input signal at its invertinginput.

The amplified and filtered first-derivative signal is fed to fourelements: a delay element 35, a peak-locating comparator 36, afalse-transition gating comparator 37, and a margin-threshold circuit38. The dotted line in FIG. 7b depicts the delayed first-derivativesignal. The amplified first-derivative signal and the delayedfirst-derivative signal feed the inverting and non-inverting inputs ofthe peak-locating comparator, respectively. As shown in FIG. 7b, thereare points in time where these signals cross; i.e., they are equal. Atthese crossing points, the output of the peak-locating comparatorchanges state. FIG. 7c depicts the output of the peak-locatingcomparator 36. The peak-locating comparator 36 feeds this output signalto a latch comparator 39.

The false-transition gating comparator 37 also receives the amplifiedfirst-derivative signal at its inverting input. A feed-back signal isconnected to the non-inverting input of the comparator 37. FIG. 7ddepicts the output of the comparator 37. If the false-transition gatingcomparator has changed state since the last change of state of thepeak-locating comparator, the latch comparator will change state. Inother words, the latch comparator only changes state upon the firsttransition of the peak-locating comparator following a transition of thegating comparator. In this way, noise that might cause spurioustransitions on the output of the peak-locating comparator do not causefalse transitions on the latch comparator output unless the noise is bigenough to trip the false-transition gating comparator.

The output of the latch comparator 39, is fed through a transistor 40 toan output 41. FIG. 7e depicts the signal at the output 41. Also, foreach point in the circuit requiring an analog ground 42, this ground isdeveloped by an auxiliary circuit 43.

The level at which the gating comparator trips is determined by theamount of hysteresis resulting from the positive feedback provided. Thislevel is set to be slightly lower than the smallest genuine signal peaksthat are expected, but higher than the typical noise levels. Thepeak-locator comparator, on the other hand, is provided with minimalhysteresis so as to insure maximum digitizing accuracy.

The margin threshold, set by the margin threshold circuit 38, is set tobe a fixed D.C. voltage below zero. The margin threshold circuit 38 actslike a retriggerable one shot multivibrator that will not time out aslong as there is a continuous series of pulses that exceed thethreshold. When the pulses stop long enough for it to time out, thedigitized bar output is forced back to the white (space) state. FIG. 7fdepicts the output of the margin threshold circuit 38. This marginthreshold circuit 38 provides increased noise immunity in the margin,i.e. the region beyond the extremes of the bar code.

Referring to FIG. 8, a technique for turning the scan motor 23 on andoff is disclosed. Usually, a trigger switch is employed so that the usercan manually initiate the scan operation by squeezing the trigger on thehandle of a hand-held scanner unit when the user is ready to read a barcode symbol. In the interest of reducing the parts count and assemblytime, and thus lowering cost and increasing reliability, the mechanicaltrigger switch is eliminated by turning the unit on in response to theunit being picked up by the operator. A motion or acceleration detectingmechanism is thus employed, so whenever the unit is laid down on thecounter by the user, the scan motor 23 and the LED light source 10 areturned off (as by a time out arrangement), but when the unit is pickedup the motion is detected and used to initiate operation of the scanner.

To this end, the coil 50 of the motor 23 is connected through a switch51 in series with the power supply 52, so the motor 23 is energized onlywhen the FET switch 51 is turned on. The voltage across the coil 50 isdetected by a detector 53, however, so that when the motor coil is notenergized, any movement of the motor shaft can be sensed due to a smallvoltage generated in the coil by movement of the rotor past the coils.The mirrors 12 and 18 are pivoted to rotate freely with the motor shaft,and any slight movement when the motor coil 50 is not energized willcause the mirrors to move. The output 54 from the detector 53 is appliedto a controller 55, and an output 56 from the controller 55 is appliedto the gate of the FET switch 51 to turn on or turn off the motor. Thecontroller 55 may be an Intel 8031 type of microcontroller, for example,and may be the controller used to evaluate the digitized bar codesignals from output 41 in FIG. 6. In addition, an output 57 from thecontroller 55 is employed to activate the LED light source 10. Anindicator light 58 (or beeper) may also be activated by the controller55 when a valid bar code is decoded, demonstrating to the user that thetask is completed. The controller 55 may also contain timing registersused to count down various time-out periods; for example, the motor andlight source may be automatically cut off after a given period since theturn-on initiated by the detector 53, or after a valid bar code has beenrecognized. If the unit is turned off by time-out when the user stillhas the housing 25 in his hand and is going to read other bar codes,then it will be turned on again due to the movement to orient it towardthe next bar code symbol.

Instead of the contact type of scanner seen in FIG. 5, the unit may beof the type held several inches or more away from the symbol to be read,as found, for example, by reference to U.S. Pat. Nos. 4,387,297;4,409,470 and 4,816,660, the entire contents of which are herebyincorporated by reference herein. The housing 25 can also beincorporated in a stand-alone workstation mounted on a countertop orsimilar support surface for use in either a scan-above, scan-below orscan-sideways system. The head can be incorporated in a fixed oradjustable mount installation.

The scanning mechanism including the pivoted mirrors 12 and 18 mountedon the shaft 60 of the motor 23 is operative for sweeping the scan spot15 across the symbol 14 in a scan, and is preferably a high-speedscanner motor of the type shown and described in U.S. Pat. No.4,387,397, the entire contents of said patent being incorporated hereinby reference and made part of the instant application. For purposes ofthis application, it is believed to be sufficient to point out that thescanner motor 23 has an output shaft 60 on which the focusing mirror 12is fixedly mounted. The motor 23 is driven to reciprocatingly andrepetitively oscillate the shaft and mirror in alternate circumferentialdirections about the axis of the shaft 60 over arc lengths of anydesired size, typically much less than 360° (The embodiment of FIGS. 1and 5 uses an angle of about 32°), and at a rate of speed on the orderof a plurality of oscillations per second. In a preferred embodiment,the focusing mirror 12 and the shaft 60 are oscillated jointly so thatthe scan spot 15 is repetitively swept in a linear scan 24 across thesymbol 14 lengthwise thereof through an angular distance or arc lengthat the scan plane 22 of about 32° and at a rate of about twenty scans toforty oscillations per second.

Although this invention is being described in connection with a singlelinear scan 24 extending across a symbol 14, it is not intended for theinvention to be so limited, since various other types of scan patternmay be formed over the symbol to be read. The scan pattern, for example,can be a set of mutually parallel linear scan lines, as set forth incopending application Ser. No. 317,533, filed Mar. 1, 1989, or in U.S.Pat. Nos. 4,369,361 or 4,387,297.

As previously mentioned, each of the bars and spaces of the symbol 14 ofthe density commonly found in the retail industry for identifying retailmerchandise measures on the order of a few mils. The scan spot 15, whichis focused by the focusing mirror 12, measures on the order of severalmillimeters and, hence, would lead to decoding errors, since the scanspot 15 is much too large to reliably detect the leading and trailingedges of each bar of the symbol. In laser-based scanners, thecross-section of the scan spot at the symbol generally measures from 6-to 10- mils, and this size is generally regarded as being optimal forminimizing decoding and reading errors without resorting to complex,highly sophisticated, signal processing circuitry or excess power loss.

Hence, in accordance with this invention, imaging means are provided inthe housing 25 for imaging a viewing window 21 of the photodiode 17, theviewing window 21 having an area smaller than that of, and locatedentirely within, the scan spot 15. The imaging means advantageouslyincludes an imaging mirror 18, and an apertured wall 19 having anaperture or slit 20 formed therethrough. The imaging mirror 18 isadvantageously mounted on the focusing mirror 12 for joint movementtherewith by the scanner motor 23 about the axis or the shaft 60. Theimaging mirror 18 is angularly offset from the axis of the mirror 12 atan angle α. The slit 20 is located immediately adjacent a sensor openingof the photodiode 17. The sensor opening can itself serve as the slit20. Alternatively, the photodiode 17 may advantageously be a very smallphotodetector. Such a small photodetector reduces noise, lowers cost ofthe scanner, and diminishes sensitivity to soiled optics. The slit 20,in a preferred embodiment, is formed with a generally rectangular orelliptical cross-section whose shorter dimension, e.g. 6-mils, islocated along the scan direction, and whose longer dimension, e.g.16-mils, is located transversely of the scan direction. The rectangularslit 20, together with the imaging mirror 18, form the viewing window 21with a similar rectangular shape, as best shown in FIG. 2. If the slit20 is the preferable elliptical shape, then the viewing window 21 willhave a similar elliptical shape.

The imaging mirror 18, which also has a generally spherical reflectingsurface, positions the viewing window 21 at a viewing plane 22 that islocated exteriorly of the housing 25. The viewing plane 22 islongitudinally spaced away from the scan plane 13. Both the focusing andimaging mirrors 12 and 18, preferably, but not necessarily, have thesame focal length. The slit 20 is imaged at the viewing plane 22 with amagnification close to one, thereby resulting in an image size of theslit of about 6-by-16 mils (the size of the viewing window 21). A firstoptical axis is concentric with the light beam 11 between the LED 10 andthe focusing mirror 12, and a second optical axis is concentric with thereturning light beam between the imaging mirror 18 and the photodiode17. The angle β included between these first and second optical axes ison the order of twice the angular offset α between the imaging andfocusing mirrors.

Hence, in accordance with this invention, the photodiode 17 "sees" onlya very small central portion of the LED scan spot 15. The image of thephotodiode slit 20 constitutes the actual scanning spot. The resultingsystem, therefore, does not have the decoding errors which would beinherent in using a multi-millimeter sized scan spot, but, instead usesa viewing window whose dimensions, at least as considered along the scandirection, are on the same order of magnitude as those of the bars andspaces of the symbol to be read.

This invention is not intended to be limited to rectangular slits, sinceother shapes are possible. The shape of the slit determines the depth offocus and the readability of the symbol.

As shown in the alternate embodiment of FIG. 9, rather than relying on asingle photodiode, a pair of photodiodes 17a and 17b are provided in thebar code reader, each photodiode and apertured wall has associated withit its own imaging mirror 18a and 18b which, in turn, form a pair ofsuperimposed viewing windows 21a and 21b as seen in FIG. 10. The viewingwindow 21a is more suited for high density bar code symbols, whereas theviewing window 21b is larger and more suited for low density bar codesymbols. The window 21b has a width corresponding to the minimum widthof features of the bar code symbol 14b. Alternatively, the viewingwindows 21a and 21b may be in different viewing planes 22, offset fromeach other as well as from the scan plane 13; thus, symbols at differentdistances from the reader unit may be brought into sharp focus, therebyextending the working range.

FIGS. 11 and 12 depict a currently preferred commercial embodiment of ascanner head in accordance with the present invention. The scanningcomponents are contained within a casing 62 which is preferably oflight-weight plastic, yet is sturdy enough to withstand the rigors ofnormal use. All major components may be mounted on a printed circuitboard 64. These major components comprise a light-emitting diode 66, aphotodetector 68, a scanning assembly 70, as well as the variouselectronic and other components to provide the scanning of symbols asdescribed herein.

The scanning assembly 70 includes a coil 72, a magnet 74, a motorsupport frame 76, a pair of leaf springs 78, preferably of Mylar, asmaller mirror 80, and a larger mirror 82. The light from the LED 66follows an axis 84 and reflected light returning to the scanner from asymbol to the photodetector follows an axis 86. As before, the anglebetween the axes 84 and 86 is β, preferably about 8°, for example. Lightfrom the LED 66, reflected by the larger mirror 82, leaves and returnsto the scanner in a scan plane 88. The angle Γ between the axis 84 andthe scan plane 88 is preferably about 7.5° to 8°, for example. As shownin FIG. 12, the LED is tilted up slightly to provide this angle, incombination with the arrangement of the larger mirror 82.

To scan a symbol, the coil 72 is pulsed with electrical power aplurality of times a second to provide on the order of 40 scans persecond, for example. When the coil 72 is energized, it attracts themagnet 74, which is attached to a mirror support frame 90. The mirrorsupport frame 90 supports the mirrors 80 and 82. Attracting the magnet74 into the coil 72 pivots the mirror support frame 90 and the mirrors80 and 82 about a pivot 92. This pivoting action creates torsion stressin the leaf springs 78 and, when the coil is de-energized, the leafsprings restore the mirror support frame and the mirrors to theirquiescent position. Pulsing the coil 72 with an electrical pulse traincoordinated with the restorative strength of the leaf springs 78provides a smooth, even scan of the desired frequency and scan width.

As shown in FIGS. 11 and 12, the scanner of the present inventionrequires no trigger as in known scanners. The scanner preferably employsthe motion sensor as described with regard to FIG. 8 which detects whenthe user picks up the hand-held unit, and turns on the light source andthe source of the electrical pulse train to the coil 72 in response tothis detection. The pulse train and light source can then be turned offby a time-out circuit, or in response to decoding a valid bar code. Themotion sensor also may sense when the user moves the scanner to a nextbar code and thereby initiate a scan.

Referring to FIG. 13, the sensor 17 is connected to circuitry forprocessing the electrical signals produced as a result of the bar codescan. This digitizer circuitry may be of the same general type disclosedin U.S. patent application Ser. No. 440,510, filed Nov. 22, 1989, orother such digitizing circuitry as set forth in the above-identifiedpatents; for example, such signal processing circuitry can be of thetype described and claimed in U.S. Pat. No. 4,360,798, incorporatedherein by reference. However, according to one feature of a preferredembodiment, an automatic bandwidth control is added. The analog signalproduced by the sensor 17 on line 94 is applied to a differentialcircuit 96 which produces an output that is the first derivative of theanalog signal. The frequency content or bandwidth of the analog signalor the first derivative signal is dependant upon the type of bar codesymbol 14 being scanned. A high density bar code 14 produces a higherfrequency of transitions between black and white and so the signal hasmore peaks and valleys, or the first derivative has more zero-crossings.The output of the differential circuit 96 is applied to a low-passfilter 98 which has a resistor 100 and a capacitor 102 connected acrossthe differentiator output. The cut-off frequency of this filter isdependant upon the value of the capacitor in series with the resistor,and so in order to switch this cut-off frequency (and thus switch thebandwidth to which the digitizing circuitry responds) and additionalcapacitor 104 is in parallel with the capacitor 102 and a switch 106provided to remove this capacitor 104 from the filter circuit. When avoltage exceeding the threshold voltage of the FET switch 106 is appliedto a line 108, the switch completes the circuit to place the capacitor104 in parallel with the capacitor 102 and thus lowers the cutofffrequency (narrows the bandwidth), but if a zero voltage is applied bythe line 108 to the gate of the FET switch then the capacitor 104 isremoved and the cut-off frequency is higher (bandwidth is wider). Theoutput 110 from the filter is applied to a zero-crossing detector 112,producing an output. This signal, again, will have a higher frequencycontent when the bar code density is higher, since the number ofzero-crossings per unit time is higher. The output of the zero-crossingdetector 112 is coupled to further processing circuitry 114 ofconventional type, producing a digitized electrical signal at output116, and this signal is either sent to the terminal unit by the cable31, or further processed to recognize and recover bar code data in thehousing 25 itself. In addition, however, the output signal of thezero-crossing detector 112 is applied to a detector 118 (e.g., anintegrator circuit and an invertor) to produce a voltage on the line 108back to the FET switch 106 that is high for low density bar codes andzero for high density bar codes. In this manner, the capacitor 104 isswitch out of the circuit when scanning high density bar codes, makingthe bandwidth of the filter wider, or the capacitor 104 is left in thefilter when scanning low density bar codes, making the bandwidthnarrower. Thus, due to the narrower bandwidth, noise is removed from theprocessing circuitry for low density bar code scanning, making it morelikely to obtain a valid bar code recognition.

Alternatively, the bandwidth switching circuitry can be responsive tothe output of the bar code recognition arrangement, i.e., downstream ofthe processor 114. The bar code signals produced at the output 116 areusually examined to see if a valid bar code is being read. A bar codecan be recognized or distinguished from noise or from text printed onthe package by various methods, or combinations of methods. For example,the number of transitions per unit of scan, or the ratio of black towhite per unit of scan, can indicate that the pattern being scanned ismost likely a bar code rather than text or other figures. Also, or inaddition, a look-up table of valid bar code patterns may be maintainedin memory and compared with the bar code signals on the output 116. Thisrecognition can be used to select broad or narrow bandwidth, based onidentifying the actual type of bar code (high density or low density)being scanned, or failure to recognize a bar code in one scan can resultin the bandwidth being switched to see if a valid code is recognized insubsequent scans.

It will be understood that each of the elements described above, or twoor more together, also may find a useful application in other types ofconstructions differing from the types described above.

While the invention has been illustrated and described as embodied in alight emitting diode scanner, it is not intended to be limited to thedetails shown, since various modifications and structural changes may bemade without departing in any way from the spirit of the presentinvention.

Without further analysis, the foregoing will so fully reveal the gist ofthe present invention that others can, by applying current knowledge,readily adapt it for the various applications without omitting featuresthat, from the standpoint of prior art, fairly constitute essentialcharacteristics of the generic or specific aspects of this inventionand, therefore, such adaptations should be and are intended to becomprehended within the meaning and range of equivalence of thefollowing claims.

What is claimed as new and desired to be protected by Letters Patent isset forth in the appended claims.

We claim:
 1. A system for reading symbols, comprising:(a) light sourcemeans for generating a light beam; (b) focusing means for focusing thelight beam to a scan spot of predetermined area at scan plane and forpositioning the scan spot on a symbol located in a working distancerange in the vicinity of the scan plane; (c) scanning means for sweepingthe scan spot across the symbol in a scan, thereby reflecting off thesymbol reflected light, at least a returning portion of which travelsaway from the symbol back in the direction of the light source, thereturning portion of the reflected light having a variable intensityover the scan; (d) a sensor for viewing and detecting the variableintensity of the returning portion of the reflected light over a fieldof view, and for generating an electrical signal indicative of thedetected variable light intensity; (e) a signal processor for processingthe electrical signal and generating therefrom data descriptive of thesymbol, the signal processor including(i) a converter for receiving theelectrical signal and developing an amplified, filtered, and invertedfirst voltage signal relative to the electrical signal, (ii) a delaycircuit for developing a delayed version of the first voltage signal(iii) a peak-locating comparator for receiving the first voltage signaland the delayed version of the first voltage signal, the peak locatingcomparator changing state whenever the first voltage signal and thedelayed version of the first voltage signal are equal, and thepeak-locating comparator generating an output, (iv) a gating comparatorfor receiving the first voltage signal, for changing state on pulses ofthe first voltage signal that are greater than positive and negativenoise thresholds, and for generating an output, and (v) a latchcomparator for receiving the outputs of the peak-locating comparator andthe gating comparator, the latch comparator changing state only if thegating comparator has changed state since the last change of state ofthe peak-locating comparator, the latch comparator providing datadescriptive of the symbol; and (f) imaging means for imaging a viewingwindow of the sensor means, said viewing window having an area smallerthan that of the scan spot, and for positioning the viewing window at aviewing plane that is spaced away from the scan plane.
 2. The systemaccording to claim 1, wherein the light source means includes anon-laser source.
 3. The system according to claim 2, wherein thenon-laser source is a light emitting diode.
 4. The system according toclaim 1, wherein the focusing means includes a focusing mirror operativeto optically form the scan spot to have a generally circular area. 5.The system according to claim 4, wherein the focusing mirror has agenerally spherical reflecting surface.
 6. The system according to claim1, wherein the sensor means includes a photodiode having a sensoropening, and wherein the imaging means includes an apertured wall havingan aperture adjacent the sensor opening and through which aperture saidreturning portion of the reflected light passes.
 7. The system accordingto claim 6, wherein the aperture has a generally rectangular shape,having a shorter dimension as considered longitudinally along thesymbol, and a longer dimension as considered in a direction transverselyof the symbol.
 8. The system according to claim 6, wherein the aperturehas a generally elliptical shape, having a shorter dimension asconsidered longitudinally along the symbol, and a longer dimension asconsidered in a direction transversely of the symbol.
 9. The systemaccording to claim 6, wherein the imaging means further includes animaging mirror.
 10. The system according to claim 9, wherein the imagingmirror has a generally spherical reflecting surface.
 11. The systemaccording to claim 1, wherein the focusing means includes a focusingmirror, and wherein the imaging means includes an imaging mirror, andwherein the mirrors are mounted in the housing for joint movementlongitudinally across the symbol.
 12. The system according to claim 11,wherein the scanning means includes a reciprocatingly oscillatablescanning component on which both the focusing mirror and the imagingmirror are mounted for joint movement herewith about an axis.
 13. Thesystem according to claim 12, wherein the imaging mirror is mounted onthe focusing mirror and angularly offset relative to said axis.
 14. Thesystem according to claim 13, wherein the light source means generatesthe light beam along a first optical axis, and wherein the imaging meansfocuses the view of the sensor means along a second optical axis, andherein the angle included between said optical axes is on the order oftwice the angular offset between the imaging and focusing mirrors. 15.The system according to claim 1, wherein the housing has a handle toenable hand-held operation, and wherein the housing is aimable by a userat the symbol to read at a longitudinal distant from a front wall, andwherein the scan plane and the viewing plane are longitudinally offsetrelative the front wall of the housing.
 16. The system according toclaim 1, wherein the sensor means includes a pair of photodiodes, eachhaving a sensor opening, and wherein the imaging means includes a pairof apertured walls, each having an aperture situated adjacent arespective sensor opening, and wherein the imaging means is operativefor focusing each view of the respective photodiodes to a pair ofviewing windows positioned in different viewing planes exteriorly of thehousing.
 17. A method of reading symbols, comprising the steps of:(a)generating a light beam; (b) focusing the light beam to a scan spot ofpredetermined area at a scan plane, the scan spot being positioned on asymbol located in a working distance range in the vicinity of the scanplane; (c) sweeping the scan spot across the symbol in a scan, at leasta returning portion of light being reflected back from the symbol, saidreturning portion of the reflected light having a variable intensityover the scan; (d) viewing and detecting the variable intensity of saidreturning portion of the reflected light over a field of view by sensormeans, and generating an electrical signal indicative of the detectedvariable light intensity; (e) processing the electrical signal andgenerating therefrom data representative of the symbol, the step ofprocessing including(i) converting the electrical signal into anamplified, filtered, and inverted first voltage relative to theelectrical signal, (ii) developing a delayed version of the firstvoltage signal, (iii) comparing the first voltage signal and the delayedversion of the first voltage signal in a peak-locating comparator, thepeak locating comparator changing state when the first voltage signaland the delayed version of the first voltage signal are equal, (iv)receiving the first voltage signal in a gating comparator, the gatingcomparator changing state when the first voltage signal exceeds positiveand negative noise thresholds, and (v) receiving the outputs of thepeak-locating comparator and gating comparator, the latch comparatorchanging state only if the gating comparator has changed state since thelast change of state of the peak-locating comparator, the latchcomparator providing data descriptive of the symbol; and (f) imaging aviewing window of the sensor means, the viewing window having an areamuch smaller than that of the scan spot, and positioning the viewingwindow at a viewing plane that is spaced away from the scan plane.
 18. Amethod according to claim 17 wherein said step of focusing includesreflecting the light beam by a curved mirror.
 19. A method according toclaim 17 wherein said step of viewing includes reflecting said returningportion by a curved mirror.
 20. A method according to claim 18 whereinsaid step of sweeping includes moving said curved mirror.
 21. A methodaccording to claim 17 wherein said viewing window has a width of aboutthe same size as a minimum dimension of features of said symbol.